In general, a reluctance machine is an electric machine in which torque is produced by the tendency of its movable part to move into a position where the reluctance of an excited winding is minimized (i.e., the inductance is maximized).
In one type of reluctance machine the energization of the phase windings occurs at a controlled frequency. These machines are generally referred to as synchronous reluctance machines. In a second type of reluctance machine, circuitry is provided for detecting the angular position of the rotor and energizing the phase windings as a function of the rotor's position. This second type of reluctance machine is generally known as a switched reluctance machine. Although the description of the present invention is in the context of a switched reluctance machine, the present invention is applicable to all forms of reluctance machines, including synchronous and switched reluctance motors, synchronous and switched reluctance generators, as well as to other machines that have phase winding arrangements similar to those of switched reluctance machines.
The general theory of design and operation of switched reluctance machines is well known and discussed, for example in The Characteristics, Design and Applications of Switched Reluctance Motors and Drives, by Stephenson and Blake and presented at the PCIM '93 Conference and Exhibition at Nuremberg, Germany, Jun. 21-24, 1993.
When a switched reluctance machine is running, including at low speeds or a standstill, the torque (and other machine performance parameters) may be adjusted by monitoring the rotor's position, energizing one or more phase windings when the rotor is at a first angular position, referred to as the "turn-on angle (T.sub.ON,)" and then de-energizing the energized windings when the rotor rotates to a second angular position, referred to as the "turn-off angle (T.sub.OFF)." The angular distance between the turn-on angle and the turn-off angle is often referred to as the "conduction angle."
At standstill and at low speeds, the torque of a switched reluctance machine can be controlled by varying the magnitude of the current in the energized phase windings over the period defined by T.sub.ON and T.sub.OFF. Such current control can be achieved by chopping the current using a current reference with phase current feedback. Such current control is referred to as "chopping mode" current control. Alternately, pulse width modulation (PWM) voltage control may be used.
As the angular speed of the motor increases, a point is reached where the amount of current which can be delivered into a phase winding during each phase period is limited by the rapidly increasing inductance and counter emf associated with the winding. At such speeds pulse width modulation or chopping strategies are less desirable and the torque of the machine is commonly controlled by controlling the duration of the voltage pulse applied to the winding during the phase period with respect to the rotor's position. Because a single pulse of voltage is applied during each phase period, this form of control is often referred to as "single-pulse control."
As a switched reluctance motor (or generator) operates, magnetic flux is continuously increasing and decreasing in different parts of the machine. This changing flux will occur in both chopping mode and single-pulse current control. The changing flux results in fluctuating magnetic forces being applied to the ferromagnetic parts of the machine. These forces can produce unwanted noise and vibration. One major mechanism by which these forces can create noise is the ovalizing of the stator caused by magnetic forces normal to the air-gap. Generally, as the magnetic flux increases along a given diameter of the stator, the stator is pulled into an oval shape by the magnetic forces. As the magnetic flux decreases, the stator pulls or springs back to its undistorted shape. This ovalizing and springing back of the stator will produce audible noise and can cause unwanted vibration.
In addition to the distortions of the stator by the ovalizing magnetic forces, acoustic noise and unwanted vibration may also be produced by abrupt changes in the magnetic forces in the motor. These abrupt changes in the gradient of the magnetic flux (i.e., the rate of change of the flux with time) are referred to as "hammer blows" because the effect on the stator is similar to that of a hammer strike. Just as a hammer strike may cause the stator to vibrate at one or more natural frequencies (determined by the mass and elasticity of the stator) the abrupt application or removal of magnetic force can cause the stator to vibrate at one or more of its natural frequencies. In general, the lowest (or fundamental) natural frequency dominates the vibration, although higher harmonics may be emphasized by repeated excitation at the appropriate frequency.
In addition to the stator distortions resulting from the ovalizing and hammer blow phenomena described above, the fluctuating magnetic forces in the motor can distort the stator in other ways, as well as distorting the rotor and other parts of the machine system. For example, distortions of the rotor can cause resonance of the rotor end-shields. These additional distortions are another potential source of unwanted vibration and noise.
Although the problem of unwanted acoustic noise and vibration has been recognized, known control systems for reluctance motors do not adequately solve the problem. For example, the general problem of acoustic noise in switched reluctance motor systems is discussed in C. Y. Wu and C. Pollock, "Analysis and Reduction of Vibration and Acoustic Noise in the Switched Reluctance Drive," Proceedings of the IAS '93 pp. 106-113 (1993). In general, the method suggested by Wu and Pollock involves control of the current in the phase winding such that the current is controlled in two successive switching steps with the second switching step occurring approximately one-half of a resonant cycle after the first where the resonant cycle is defined by the natural frequency of the machine. This approach is typically implemented by switching off one of the power devices at a first point in time to cause a first stepped reduction in applied voltage, and then later switching off the second power device. Between the time when the first switching device is switched off and the second switching device is switched off, the current is allowed to freewheel through a freewheeling diode and the second switching device.
The two-step voltage-reduction approach to noise reduction in switched reluctance motors discussed above suffers from several limitations and disadvantages. One such limitation is that in many cases the two-step voltage-reduction approach requires precise switching of the switching devices within the interval defined by the turn-on and turn-off angles (i.e., the angular interval during which the phase winding is energized). Still further, the two-step voltage-reduction approach limits the flexibility to dynamically adjust the freewheeling period for each phase cycle. As discussed above, in the two-step voltage-reduction approach, the duration of the freewheeling period is optimized to reduce the noise produced by the system at a single fundamental frequency. There are many instances when it would be desirable to optimize the freewheeling duration according to other criteria.
An additional limitation of the two-step voltage-reduction approach, and other approaches that utilize freewheeling to reduce noise, is that, since there is typically only one freewheeling period per phase energization cycle, freewheeling generally reduces noise produced by only a single frequency of the motor system. Freewheeling to reduce noise at one frequency does not reduce noise produced at other frequencies, in motor systems that have multiple resonant frequencies. Accordingly, such approaches do not reduce many sources of unwanted noise. A further disadvantage with the freewheeling approaches is that there are several motor control systems (e.g., H-circuits with a split capacitor, third-rail circuits, ring circuits and the like) that simply do not allow freewheeling. These systems cannot use freewheeling to reduce noise.
The present invention overcomes many of the limitations and disadvantages associated with known systems and provides a unique method and apparatus for controlling the phase voltage and the phase winding currents in a phase windings of a switched reluctance machine to, for example, reduce unwanted machine noise and vibrations.